1. Field of the Invention
This invention is concerned with the removal of organic cations from organic cation-containing molecular sieves or zeolites. More specifically, this invention is directed to a chemical treatment of organic cation-containing zeolites to facilitate removal of the organic cation from the zeolite and to render it more suitable for subsequent ion exchange, and/or improve the dispersive and catalytic properties thereof.
2. Discussion of the Prior Art
Molecular sieves or zeolites have, for some time, been known to be crystalline and to have a rigid three-dimensional structure wherein the pores of the material are uniform. These materials, especially when ion exchanged, are useful in the field of catalysis, especially for the conversion of hydrocarbons.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. These materials have come to be designated by letters or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), zeolite ZSM-35 (U.S. Pat. No. 4,016,245), zeolite ZSM-21 and 38 (U.S. Pat. No. 4,046,859), and zeolite ZSM-23 (U.S. Pat. No. 4,076,842) merely to name a few.
Certain zeolites function desirably as shape selective catalysts. Zeolites are generally useful for alkylation, polymerization (depending upon pore size of the molecular sieve), isomerization of aliphatics and aromatics, dealkylation and reforming. More recently, molecular sieves have been prepared by including in the reaction mixture an organic cation. A typical type of organic cation is a quaternary ammonium cation, such as tetramethylammonium. It is supplied to the reaction mixture in the form of a salt such as tetramethylammonium chloride or tetramethylammonium sulfate. Additionally, it can be supplied to the reaction mixture in alkaline form as tetramethylammonium hydroxide. It has become desirable to convert the as-synthesized form of these organic cation-containing zeolites to remove the organic cation and through base exchange and calcination to convert the same to a more highly catalytically active form. Heretofore, the organic cation has been removed by subjecting the zeolite as synthesized to an elevated temperature, such treatment being referred to as precalcination. Thereafter, the zeolite is treated in accordance with known techniques to convert it to the desired form through base exchange and final calcination.
Unfortunately, the precalcination procedure heretofore employed has required high temperatures which are not generally beneficial to the crystallinity of the zeolite material. Additionally, such high temperature precalcination, at temperatures ordinarily between 700.degree. and 1200.degree. F., adds to the cost of converting the organic cation-containing zeolite to a more preferably catalytically-active form.
In the preparation of zeolite containing composite catalysts, i.e., zeolite plus a binder or matrix, it is necessary that the procedure transform the catalytic composite to the active catalytic form without damaging or impairing the catalytic potential of the composite. It is also important that the dispersion in the catalytic composite be such that uniformity of catalyst composition, good physical properties and high utilization of the catalytic component (usually the zeolite) are attained.
In the preparation of catalysts employing zeolite components that have been crystallized with and contain organic components either as cations or occluded material, it is necessary to remove the organic compound so that other cations, such as sodium, can be removed and replaced with cations that will result in an active catalytic form. In cases where other cations are not present in amounts sufficient to impair catalytic activity, the organic cations must nevertheless be removed to make the catalytically active form. In either case, the organic material must be removed in such a manner as not to damage the zeolite, such as thermal damage if the organics are oxidized too rapidly in an air calcination.
Two methods have been used to remove the organic material from zeolites. One conventional method involves calcination in a non-oxidizing atmosphere, e.g., N.sub.2, NH.sub.3, steam, etc. The second method involves an aqueous liquid phase oxidation at elevated temperatures as covered in U.S. Pat. No. 3,766,093.
When zeolite containing catalysts are prepared in form for use in a continuous fluidized bed process, final calcination is not necessarily required, since the fresh catalyst make-up is usually added to the high temperature regenerator portion of the process and calcination to remove water and other volatiles is effectively done in the regenerator. With catalysts, which are comprised of organic containing zeolites, adding uncalcined catalyst to the regenerator would probably result in thermal damage to the zeolite due to the excessive heat released as the organics are burned.